The present invention relates to an offset compensation circuit and method and, particularly, to an offset compensation circuit and method which enable offset compensation with a simple configuration.
It is known that television broadcasting schemes of satellite broadcasting and CATV digitally transmit television signals or the like by using modulation techniques such as QPSK and QAM. The receiving circuits receiving the broadcasting radio waves of those techniques convert a received analog signal to digital and perform digital processing on the resulting signal.
In demodulating a transmission signal in the above systems, the center value of the amplitude of a reception signal does not necessarily coincide with the ideal center value because of variations in the resistance of external resistors that relate to the A/D conversion. For example, if there is a 2% to 3% variation as a DC offset, an A/D converter having nominal resolution of 8 bits can only exhibit resolution lower than 8 bits. Therefore, in receiving circuits, a DC offset is detected from actual demodulation data and compensation is made therefor.
One method for detecting a DC offset is to sample the A/D-converted data, calculate an average, and employ the average as a DC offset value.
Another method is to determine the maximum and minimum values of data obtained by actually converting a reception signal from analog to digital. The deviation of the center of the maximum and minimum values from the center of the dynamic range of an A/D converter is used as a DC offset value. A further method is to extract only high-frequency components using a high-pass digital filter.
In the above described method of detecting a DC offset by averaging outputs of an A/D converter, it is necessary to average large numbers (ten or more) of data because even an analog signal has a DC component in a short period. This method therefore has a problem of a large circuit scale, as does the above method of using a digital filter.
Furthermore, the above described method of determining a DC offset from the center of the maximum and minimum values of an A/D converted reception signal and the center of a dynamic range is disadvantageous because it is easily affected by noise. Therefore, it is difficult to apply this method to an analog signal (transmission signal) containing a large amount of noise components.